High-level Programming with Python Expressions and Variables Alex Ropelewski PSC-NRBSC Bienvenido Vélez UPR Mayaguez Reference: How to Think Like a Computer Scientist: Learning with Python Essential Computing for Bioinformatics 1

The following material is the result of a curriculum development effort to provide a set of courses to support bioinformatics efforts involving students from the biological sciences, computer science, and mathematics departments. They have been developed as a part of the NIH funded project “Assisting Bioinformatics Efforts at Minority Schools” (2T36 GM008789). The people involved with the curriculum development effort include: Dr. Hugh B. Nicholas, Dr. Troy Wymore, Mr. Alexander Ropelewski and Dr. David Deerfield II, National Resource for Biomedical Supercomputing, Pittsburgh Supercomputing Center, Carnegie Mellon University. Dr. Ricardo González Méndez, University of Puerto Rico Medical Sciences Campus. Dr. Alade Tokuta, North Carolina Central University. Dr. Jaime Seguel and Dr. Bienvenido Vélez, University of Puerto Rico at Mayagüez. Dr. Satish Bhalla, Johnson C. Smith University. Unless otherwise specified, all the information contained within is Copyrighted © by Carnegie Mellon University. Permission is granted for use, modify, and reproduce these materials for teaching purposes. Most recent versions of these presentations can be found at

Integer Expressions 3 >>> print >>> print 6 * 7 42 >>> print 6 * >>> print * 7 44 >>> print >>> print 6 - ( 2 - 3) 7 >>> print 1 / 3 0 >>> / and * higher precedence than + and - integer division truncates fractional part Operators are left associative Parenthesis can override precedence

Floating Point Expressions 4 >>> print 1.0 / >>> print >>> print 3.3 * >>> print 3.3e23 * 2 6.6e+023 >>> print float(1) / >>> Mixed operations converted to float Scientific notation allowed 12 decimal digits default precision Explicit conversion

String Expressions 5 >>> print "aaa" aaa >>> print "aaa" + "ccc" aaaccc >>> len("aaa") 3 >>> len ("aaa" + "ccc") 6 >>> print "aaa" * 4 aaaaaaaaaaaa >>> "aaa" 'aaa' >>> "c" in "atc" True >>> "g" in "atc" False >>> + concatenates string len is a function that returns the length of its argument string any expression can be an argument * replicates strings a value is an expression that yields itself in operator finds a string inside another And returns a boolean result

MEANINGFUL Values Can Have (MEANINGFUL) Names 6 >>> numAminoAcids = 20 >>> eValue = 6.022e23 >>> prompt = "Enter a sequence ->" >>> print numAminoAcids 20 >>> print eValue 6.022e+023 >>> print prompt Enter a sequence -> >>> print "prompt" prompt >>> >>> prompt = 5 >>> print prompt 5 >>> = binds a name to a value prints the value bound to a name = can change the value associated with a name even to a different type use Camel case for compound names

Values Have Types 7 >>> type("hello") >>> type(3) >>> type(3.0) >>> type(eValue) >>> type (prompt) >>> type(numAminoAcids) >>> type is another function the type of a name is the type of the value bound to it the “type” is itself a value

In Bioinformatics Words … 8 >>> codon=“atg” >>> codon * 3 ’atgatgatg’ >>> seq1 =“agcgccttgaattcggcaccaggcaaatctcaaggagaagttccggggagaaggtgaaga” >>> seq2 = “cggggagtggggagttgagtcgcaagatgagcgagcggatgtccactatgagcgataata” >>> seq = seq1 + seq2 >>> seq 'agcgccttgaattcggcaccaggcaaatctcaaggagaagttccggggagaaggtgaagacggggagtggggagttgagtc gcaagatgagcgagcggatgtccactatgagcgataata‘ >>> seq[1] 'g' >>> seq[0] 'a' >>> “a” in seq True >>> len(seq1) 60 >>> len(seq) 120 First nucleotide starts at 0

More Bioinformatics Extracting Information from Sequences 9 >>> seq[0] + seq[1] + seq[2] ’agc’ >>> seq[0:3] ’agc’ >>> seq[3:6] ’gcc’ >>> seq.count(’a’) 35 >>> seq.count(’c’) 21 >>> seq.count(’g’) 44 >>> seq.count(’t’) 12 >>> long = len(seq) >>> pctA = seq.count(’a’) >>> float(pctA) / long * Find the first codon from the sequence get ’slices’ from strings: How many of each base does this sequence contain? Count the percentage of each base on the sequence.

Additional Note About Python Strings 10 >>> seq=“ACGT” >>> print seq ACGT >>> seq=“TATATA” >>> print seq TATATA >>> seq[0] = seq[1] Traceback (most recent call last): File " ", line 1, in seq[0]=seq[1] TypeError: 'str' object does not support item assignment seq = seq[1] + seq[1:] Can replace one whole string with another whole string Can NOT simply replace a sequence character with another sequence character, but… Can replace a whole string using substrings